Project abstract. Activation of the renin-angiotensin system (RAS) system is associated with increased cardiovascular death. A critical component of this system is angiotensin converting enzyme (ACE), which cleaves the decapeptide angiotensin I, producing the eight amino acid peptide angiotensin II (AngII), a central signaling molecule of the RAS system. In humans, increased AngII levels are associated with an increased ventricular arrhythmic risk, and use of ACE inhibitors reduces that risk. To investigate RAS induced arrhythmias, we developed a cardiac-restricted ACE overexpression mouse that shows an increased risk of sudden death in the absence of heart failure or structural heart disease. In this application, we show that AngII- mediated oxidative stress activates the transcription factor NFkB via H2O2 production, c-Src is transcriptionally upregulated in our model, and ACE mice have reduced Cx43 protein in the absence of changes in mRNA abundance. Therefore, we will test to what extent AngII leads to oxidative stress which in-turn alters Cx43, contributing to the ACE 8/8 arrhythmic phenotype. This will be tested in 4 aims. In each aim, we will establish to what extent measures of oxidative stress, NFB, c-Src, Cx43, and arrhythmic risk are altered by the disruptions in the proposed signaling cascade. The aims are constructed to test a specific, plausible signaling cascade and simultaneously establish the order of the intermediates in the cascade. Specific aim 1: To establish to what extent AngII-mediated signaling is responsible for the Cx43 regulation in our ACE overexpression model. Specific aim 2: To establish to what extent NADPH oxidase activation is responsible for the Cx43 regulation in our ACE overexpression model. Specific aim 3: To establish to what extent increased NFkB activation is responsible for the Cx43 regulation in our ACE overexpression model. Specific aim 4: To establish to what extent increased c-Src upregulation is responsible for the Cx43 regulation in our ACE overexpression model.